1. Field of the Invention
The present invention is directed to a rechargeable battery pack for a power tool, and is more particularly to a battery pack with a self-contained over-charge protection.
2. Description of the Related Art
A rechargeable battery pack has been widely utilized for a power tool. Recently, lithium-ion batteries are becoming prevalent over conventional nickel-cadmium and nickel-hydrogen batteries because of their high power generating capability and also their light weight structure. In order to safely utilize a lithium-ion battery for the power tool, it is required to make a protection against over-charging of the battery, because the lithium-ion battery is especially susceptible to an over-charging and may rapidly deteriorate by the over-charging.
In order to avoid the over-charging, Japanese Patent Publication JP 2005-218174 A and JP2005-131770 A propose a combination of a battery pack and a charger. The battery pack has a battery stack composed of a series connected secondary cells, and incorporates a voltage monitor which is configured to detect a cell voltage across each one of the secondary cells so as to provide a detection signal indicative of the cell voltage to the charger and the power tool. The charger includes a charging circuit supplying a charge current to the battery stack based upon the detection signal so as to limit the charge current when the detection signal indicates that any one of the cells is over-charged. However, because the over-charging protection relies upon a current controller provided on the side of the charger, the protection cannot work if the charger becomes out of order. Thus, the battery may be accidentally over-charged as a consequence of the failure of the charger.